Nano graphene platelet-base composite anode compositions for lithium ion batteries

ABSTRACT

The present invention provides a nano-scaled graphene platelet-based composite material composition for use as an electrode, particularly as an anode of a lithium ion battery. The composition comprises: (a) micron- or nanometer-scaled particles or coating which are capable of absorbing and desorbing lithium ions; and (b) a plurality of nano-scaled graphene platelets (NGPs), wherein a platelet comprises a graphene sheet or a stack of graphene sheets having a platelet thickness less than 100 nm; wherein at least one of the particles or coating is physically attached or chemically bonded to at least one of the graphene platelets and the amount of platelets is in the range of 2% to 90% by weight and the amount of particles or coating in the range of 98% to 10% by weight. Also provided is a lithium secondary battery comprising such a negative electrode (anode). The battery exhibits an exceptional specific capacity, an excellent reversible capacity, and a long cycle life.

This invention is based on the research result of a U.S. FederalGovernment Small Business Innovation Research (SBIR) project. The U.S.government has certain rights on this invention.

This is a co-pending application of Aruna Zhamu and Bor Z. Jang, “HYBRIDANODE COMPOSITIONS FOR LITHIUM ION BATTERIES,” submitted to the U.S.Patent and Trademark Office on the same day as the instant application.

FIELD OF THE INVENTION

The present invention provides a nano-scaled graphene platelet-basedcomposite material for use as an anode active material in a secondarybattery, particularly lithium-ion battery.

BACKGROUND

The description of prior art will be primarily based on the list ofreferences presented at the end of this section.

Concerns over the safety of earlier lithium secondary batteries led tothe development of lithium ion secondary batteries, in which purelithium metal sheet or film was replaced by carbonaceous materials asthe anode. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1. In order to minimize the loss in energy densitydue to this replacement, x in Li_(x)C₆ must be maximized and theirreversible capacity loss Q_(ir) in the first charge of the batterymust be minimized. Carbon anodes can have a long cycle life due to thepresence of a protective surface-electrolyte interface layer (SEI),which results from the reaction between lithium and the electrolyteduring the first several cycles of charge-discharge. The lithium in thisreaction comes from some of the lithium ions originally intended for thecharge transfer purpose. As the SEI is formed, the lithium ions becomepart of the inert SEI layer and become irreversible, i.e, they can nolonger be the active element for charge transfer. Therefore, it isdesirable to use a minimum amount of lithium for the formation of aneffective SEI layer. In addition to SEI formation, Q_(ir) has beenattributed to graphite exfoliation caused by electrolyte solventco-intercalation and other side reactions [Refs. 1-4].

The maximum amount of lithium that can be reversibly intercalated intothe interstices between graphene planes of a perfect graphite crystal isgenerally believed to occur in a graphite intercalation compoundrepresented by Li_(x)C₆ (x=1), corresponding to a theoretical specificcapacity of 372 mAh/g. In other graphitized carbon materials than puregraphite crystals, there exists a certain amount of graphitecrystallites dispersed in or bonded by an amorphous or disordered carbonmatrix phase. The amorphous phase typically can store lithium to aspecific capacity level higher than 372 mAh/g, up to 700 mAh/g in somecases, although a specific capacity higher than 1,000 mAh/g has beensporadically reported. Hence, the magnitude of x in a carbonaceousmaterial Li_(x)C₆ varies with the proportion of graphite crystallitesand can be manipulated by using different processing conditions, asexemplified in [Refs. 1-4]. An amorphous carbon phase alone tends toexhibit a low electrical conductivity (high charge transfer resistance)and, hence, a high polarization or internal power loss. Conventionalamorphous carbon-based anode materials also tend to give rise to a highirreversible capacity.

The so-called “amorphous carbons” commonly used as anode activematerials are typically not purely amorphous, but contain some micro- ornano-crystallites with each crystallite being composed of a small numberof graphene sheets (basal planes) that are stacked and bonded togetherby weak van der Waals forces. The number of graphene sheets variesbetween one and several hundreds, giving rise to a c-directionaldimension (thickness Lc) of typically 0.34 nm to 100 nm. The length orwidth (La) of these crystallites is typically between tens of nanometersto microns. Among this class of carbon materials, soft and hard carbonsmade by low-temperature pyrolysis (550-1,000° C.) exhibit a reversiblecapacity of 400-800 mAh/g in the 0-2.5 V range [Refs. 1-3]. Dahn et al.have made the so-called house-of-cards carbonaceous material withenhanced capacities approaching 700 mAh/g [Refs. 1,2]. Tarascon'sresearch group obtained enhanced capacities of up to 700 mAh/g bymilling graphite, coke, or carbon fibers [Ref. 3]. Dahn et al. explainedthe origin of the extra capacity with the assumption that in disorderedcarbon containing some dispersed graphene sheets (referred to ashouse-of-cards materials), lithium ions are adsorbed on two sides of asingle graphene sheet [Refs. 1,2]. It was also proposed that Li readilybonded to a proton-passivated carbon, resulting in a series ofedge-oriented Li—C—H bonds. This provides an additional source of Li⁺ insome disordered carbons [Ref. 5]. Other researchers suggested theformation of Li metal mono-layers on the outer graphene sheets [Ref. 6]of graphite nano-crystallites. The amorphous carbons of Dahn et al. wereprepared by pyrolyzing epoxy resins and may be more correctly referredto as polymeric carbons. Polymeric carbon-based anode materials werealso studied by Zhang, et al. [Ref. 8] and Liu, et al. [Ref. 9].

Peled and co-workers improved the reversible capacity of a graphiteelectrode to ˜400 mAh/g by mild air oxidation [Ref. 4]. They showed thatmild oxidation (burning) of graphite produces well-defined voids ornano-channels, having an opening of a few nanometers and up to tens ofnanometers, on the surface of the graphite. They believed that thesenano-channels were small enough to prevent co-intercalation of thesolvent molecule but large enough to allow Li-ion penetration [Ref. 4].These nano-channels were formed at the La-Lc interface, called “zigzagand armchair faces” between two adjacent crystallites, and in thevicinity of defects and impurities. Both natural and synthetic graphitematerials typically have a wide variety of functional groups (e.g.,carbonate, hydrogen, carboxyl, lactone, phenol, carbonyl, ether, pyrone,and chromene) at the edges of crystallites defined by La and Lc [Ref.7]. These groups can react with lithium and/or electrolyte species toform a so-called in situ CB-SEI (chemically bonded solid electrolyteinterface) [Ref. 4] on which, for example, carboxylic acid surface filmsare converted into Li-carboxylic salts.

In summary, in addition to the above-cited three mechanisms, thefollowing mechanisms for the extra capacity over the theoretical valueof 372 mAh/g have been proposed [Ref. 4]: (i) lithium can occupy nearestneighbor sites; (ii) insertion of lithium species into nano-scaledcavities; (iii) in very disordered carbons containing large fractions ofsingle graphene sheets (like the structure of a house of cards) lithiummay be adsorbed on both sides of single layer sheets [Refs. 1,2]; (iv)correlation of H/C ratio with excess capacity led to a proposal thatlithium may be bound somehow in the vicinity of the hydrogen atoms(possible formation of multi-layers of lithium on the external grapheneplanes of each crystallite in disordered carbons) [Ref. 6]; and (vi)accommodation of lithium in the zigzag and armchair sites [Ref. 4].

In addition to carbon- or graphite-based anode materials, otherinorganic materials that have been evaluated for potential anodeapplications include metal oxides, metal nitrides, metal sulfides, andthe like, and a range of metals, metal alloys, and intermetalliccompounds that can accommodate lithium atoms/ions. In particular,lithium alloys having a composition formula of Li_(a)A (A is a metalsuch as Al, and “a” satisfies 0<a≦5) has been investigated as potentialanode materials. This class of anode material has a higher theoreticalcapacity, e.g., Li₄Si (3,829 mAh/g), Li_(4.4)Si (4,200 mAh/g),Li_(4.4)Ge (1,623 mAh/g), Li_(4.4)Sn (993 mAh/g), Li₃Cd (715 mAh/g),Li₃Sb (660 mAh/g), Li_(4.4)Pb (569 mAh/g), LiZn (410 mAh/g), and Li₃Bi(385 mAh/g). However, for the anodes composed of these materials,pulverization (fragmentation of the alloy particles) proceeds with theprogress of the charging and discharging cycles due to expansion andcontraction of the anode during the absorption and desorption of thelithium ions. The expansion and contraction also tend to result inreduction in or loss of particle-to-particle contacts or contactsbetween the anode and its current collector. These adverse effectsresult in a significantly shortened charge-discharge cycle life.

To overcome the problems associated with such mechanical degradation,composites composed of small electrochemically active particlessupported with less active or non-active matrices have been proposed foruse as an anode material. Examples of these active particles are Si, Sn,and SnO₂. However, most of prior art composite electrodes havedeficiencies in some ways, e.g., in most cases, less than satisfactoryreversible capacity, poor cycling stability, high irreversible capacity,ineffectiveness in reducing the internal stress or strain during thelithium ion insertion and extraction steps, and/or undesirable sideeffects.

For instance, as disclosed in U.S. Pat. No. 6,007,945 (Dec. 28, 1999) byJacobs, et al., a solid solution of titanium dioxide and tin dioxide wasutilized as the anode active substance in the negative electrode of arechargeable lithium battery. The density of the negative electrode madeas described above was 3.65 g/cm³, and the reversible capacity of thenegative electrode containing TiO₂—SnO₂ in a ratio of 39:61 by weight,was found to be 1130 mAh/cm³. This was equivalent to 309.6 mAh/g,although the obtained rechargeable lithium battery was calculated tohave energy density of 207 watt.hour per liter. Furthermore, thenanoparticles of the anode material react with the electrolyte duringthe charge-discharge cycles, resulting in reduced long-term utility.

As described in U.S. Pat. No. 6,143,448 (Nov. 7, 2000), by Fauteux etal., a composite was formed by mixing carbon with a metal salt in water,followed by evaporation, heating and further treatment. The processproduces a composite with many pores, which are not always preferred.The best achievable capacity was reported to be in the range of750-2,000 mAh/cm³. With a density of 4 g/cm³, this implies a maximumcapacity of 500 mAh/g

In U.S. Pat. No. 6,103,393 (Aug. 15, 2000), Kodas et al. providedcarbon-metal particles by mixing the reactant, making the mixture intoan aerosol, and then heating. Every particle contains a carbon phase anda metal phase. This study was primarily on carbon-supported platinum,silver, palladium, ruthenium, osmium and alloys thereof, which are forelectro-catalysis purpose (e.g., for fuel cell applications).

In U.S. Pat. No. 7,094,499 (Aug. 22, 2006), Hung disclosed a method offorming a composite anode material. The steps include selecting a carbonmaterial as a constituent part of the composite, chemically treating theselected carbon material to receive nanoparticles, incorporatingnanoparticles into the chemically treated carbon material, and removingsurface nanoparticles from an outside surface of the carbon materialwith incorporated nanoparticles. A material making up the nanoparticlesalloys with lithium. The resulting carbon/nanoparticle composite anodesdid not exhibit any significant increase in capacity, mostly lower than400 mAh/g.

In summary, the prior art has not demonstrated a composite material thathas all or most of the properties desired for use in an anode for thelithium-ion battery. Thus, there is a need for a new anode forlithium-ion batteries that has a high cycle life, high reversiblecapacity, and low irreversible capacity. There is also a need for amethod of readily or easily producing such a material.

REFERENCES

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SUMMARY OF THE INVENTION

The present invention provides a negative electrode (anode) compositematerial composition for use in a lithium secondary battery. Thecomposition comprises an electrochemically active material admixed withnano-scaled graphene platelets (NGPs), characterized in that both theactive material and the NGPs are capable of absorbing and desorbinglithium ions. The electrochemically active material is in a fine powderform (smaller than 500 μm, preferably smaller than 200 μm, and mostpreferably smaller than 1 μm) and/or thin film (coating) form(preferably smaller than 100 nm in thickness), in contact with orattached to graphene platelets.

An NGP is essentially composed of a sheet of graphene plane or multiplesheets of graphene plane stacked and bonded together through van derWaals forces. Each graphene plane, also referred to as a graphene sheetor basal plane, comprises a two-dimensional hexagonal structure ofcarbon atoms. Each plate has a length and a width parallel to thegraphite plane and a thickness orthogonal to the graphite plane. Bydefinition, the thickness of an NGP is 100 nanometers (nm) or smaller,with a single-sheet NGP being as thin as 0.34 nm. The length and widthof a NGP are typically between 1 μm and 20 μm, but could be longer orshorter. The NGPs form a myriad of electron transport paths forimproving the electrical conductivity or reducing the internalresistance of the anode. The flexibility and strength of NGPs make themideal materials to absorb or buffer the volume expansion or contractionof the electrochemically active material particles or coating. NGPsthemselves are also capable of absorbing and extracting lithium(explained in a later section).

The electrochemically active material in the present invention can beselected from the following groups of materials:

-   -   (a) Silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony        (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), and cadmium (Cd);        preferably of nanocrystalline or amorphous structure in a thin        film (coating) or micron- or nanometer-sized particulate form.        The coating is preferably thinner than 10 μm and more preferably        thinner than 1 μm;    -   (b) The alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb,        Bi, Zn, Al, or Cd, stoichiometric or non-stoichiometric with        other elements;    -   (c) The oxides, carbides, nitrides, sulfides, phosphides,        selenides, tellurides, antimonides, or their mixtures (e.g.,        co-oxides or composite oxides) of Si, Ge, Sn, Pb, Sb, Bi, Zn,        Al, Fe, or Cd. For instance, SnO or SnO₂ may be admixed with        oxides of B, Al, P, Si, Ge, Ti, Mn, Fe, or Zn and then subjected        to heat treatments to obtain composite oxides. Composite oxides        may also be prepared by mechanical alloying (e.g., ball milling        of a mixture of SnO and B₂O₃). SnO or SnO₂ alone is of        particular interest due to their high theoretical capacities.        Iron oxide or phosphate is of interest since Li₆Fe₂O₃ has a        theoretical capacity of 1,000 mAh/g. The first cycle capacity of        Fe₃PO₇ is found to reach 800 mAh/g. The capacity of SnS₂ is as        high as 620 mAh/g and is stable under charge-discharge cycling        conditions.    -   (d) Salts or hydroxides of Sn, e.g., SnSO₄ (with a reversible of        600 mAh/g), Sn₂PO₄Cl, (300 mAh/g even after 40 cycles), and        Sn₃O₂(OH)₂ (300 mAh/g).

Optionally, amorphous carbon or polymeric carbon can be incorporated, inaddition to any material in (a)-(d), in the exfoliated graphite host.The composite material (or its constituent active material and NGPsindependently) may be mixed with a resin to form a precursor composite.This precursor composite may be heated at a temperature of typically500-1,200° C. to convert the resin into a polymeric carbon or anamorphous carbon phase. Hence, in the presently invented negativeelectrode material composition, the composite material may furthercomprise an amorphous carbon phase or polymeric carbon. Alternatively,the amorphous carbon phase may be obtained from chemical vapordeposition, chemical vapor infiltration, or pyrolyzation of an organicprecursor.

The electrochemically active materials listed in (a)-(d) above, whenused alone as an anode material (with or without a polymer binder) in aparticulate or thin film form, are found to suffer from thefragmentation problem and poor cycling stability. When mixed with theNGPs to form a composite material, the resulting anode exhibits areversible capacity much higher than that of graphite (372 mAh/g), a lowirreversible capacity loss, low internal resistance, and fastcharge-recharge processes. It seems that NGPs, being mechanicallyflexible, are capable of accommodating or cushioning the strains orstresses induced by lithium insertion and extraction. With the activematerial particle size or coating thickness less than 1 μm, the distancefor lithium ions to travel is reduced. The anode can quickly store orrelease lithium and thus can carry high current. This is a highlybeneficial feature for a battery that is intended for high power densityapplications such as electric cars. NGPs also serve to separate orisolate active material particles from one another, preventingcoalescence or sintering of fine particles. Furthermore, when the amountreaches a threshold volume fraction (percolation condition), NGPs form acontinuous path for electrons, resulting in significantly reducedinternal energy loss or internal heating.

The nano-scaled graphene platelets may be obtained from intercalation,exfoliation, and separation of graphene sheets in a laminar graphitematerial selected from natural graphite, synthetic graphite, highlyoriented pyrolytic graphite, graphite fiber, carbon fiber, carbonnano-fiber, graphitic nano-fiber, spherical graphite or graphiteglobule, meso-phase micro-bead, meso-phase pitch, graphitic coke, orpolymeric carbon. For instance, natural graphite may be subjected to anintercalation/oxidation treatment under a condition comparable to whathas been commonly employed to prepare the so-called expandable graphiteor stable graphite intercalation compound (GIC). This can beaccomplished, for instance, by immersing graphite powder in a solutionof sulfuric acid, nitric acid, and potassium permanganate for preferably2-24 hours (details to be described later). The subsequently driedproduct, a GIC, is then subjected to a thermal shock (e.g., 1,000° C.for 15-30 seconds) to obtain exfoliated graphite worms, which arenetworks of interconnected exfoliated graphite flakes with each flakecomprising one or a multiplicity of graphene sheets. The exfoliatedgraphite is then subjected to mechanical shearing (e.g., using an airmilling, ball milling, or ultrasonication treatment) to break up theexfoliated graphite flakes and separate the graphene sheets {Refs.36-49]. These NGPs can be mixed with electrochemically active materialparticles, or the platelet surfaces may be deposited with a coating ofthe active material.

It may be noted that exfoliated graphite worms and NGPs are quitedistinct in structure, morphology, and properties. For instance,exfoliated graphite worms are characterized by having networks oflargely interconnected graphite flakes having pores or empty pocketsbetween flakes. The densities of worms are between 0.01 g/cm³ and 2.0g/cm³. These flakes have a typical length/width/diameter dimension of0.5-100 μm (more typically 1-20 μm) and typical thickness of 0.34 nm-500nm (more typically 10-100 nm). These flakes are still physicallyattached to or chemically bonded to one another. Upon separation andsome further exfoliation of graphene sheets that constitute the flakes,the networks are broken, the flakes are separated, and some flakes arefurther split into thinner platelets. Hence, the NGPs typically are muchthinner than their parent flakes and often smaller inlength/width/diameter. Individual NGPs have a density of approximately2.3 g/cm³. We have found that, although NGPs have been physicallyseparated from one another, they can be dispersed (mixed with anothermaterial) in such a manner that only a small weight fraction of NGPswould be sufficient to reach a percolation state that establishes anetwork of electron-conducting paths. Separated NGPs also enable moreconvenient and often more uniform mixing between platelets and otheringredients, such as the electrochemically active particles or coatingcited in the present invention. For these and other reasons, NGPs andgraphite worms are patently distinct and, hence, the instant applicationis patently distinct from the co-pending application: Aruna Zhamu andBor Z. Jang, “HYBRID ANODE COMPOSITIONS FOR LITHIUM ION BATTERIES,”submitted to the U.S. Patent and Trademark Office on the same day.

In the preparation of a negative electrode material, typicallyelectrochemically active particles are held together by the NGPs if theparticle-NGP mixture is slightly compressed. Although not a necessarycondition, a binder material may be used to further bond the particlestogether to produce an integral anode member. The binder material may bea non-conductive material, such as polyvinylidene fluoride (PVDF) andpoly(tetrafluoroethylene) (PTFE). An electrically conductive bindermaterial may be selected from coal tar pitch, petroleum pitch,meso-phase pitch, coke, a pyrolized version of pitch or coke, or aconjugate chain polymer (intrinsically conductive polymer such aspolythiophene, polypyrrole, or polyaniline.

Another preferred embodiment of the present invention is a lithiumsecondary battery comprising a positive electrode, a negative electrode,and a non-aqueous electrolyte disposed between the negative electrodeand the positive electrode. The anode (negative electrode) comprises acomposite composition composed of an electrochemically active materialadmixed with NGPs, characterized in that both the active material andthe NGPs are capable of absorbing and desorbing lithium ions. The activematerial can be in the form of fine particles or thin coating film. Theanode material in the present invention provides a reversible specificcapacity of typically greater than 600 mAh/g, often greater than 800mAh/g, and, in many cases, much greater than 1,000 mAh/g (all based onper gram of composite material), which all far exceed the theoreticalspecific capacity of 372 mAh/g for graphite anode material. They alsoexhibit superior multiple-cycle behaviors with a small capacity fade anda long cycle life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a cylinder-shape lithium ion battery.

FIG. 2 TEM micrographs of NGPs (several NGPs overlapping each other).

FIG. 3 Specific capacities of various anode materials: Sn-only,NGPs-only, theoretical model (based on a rule-of-mixture law), andSn+NGPs.

FIG. 4 Specific capacities of Si thin film anode supported on NGPs and abaseline Si thin film anode supported on a Ni foil, respectively.

FIG. 5 Specific capacities of a (Sn+Li₂O) mixture bonded by a resinbinder and that supported by NGPs, respectively.

FIG. 6 Four sets of specific capacity data: Series 1 (denoted by ♦) isfor C-coated Si particles bonded by a resin binder with specificcapacity calculated on the basis of per gram of Si (carbon weight notcounted); Series 2 (denoted by ▪) is for C-coated Si particles hosted by20% by weight of NGPs with specific capacity calculated on the basis ofper gram of Si (carbon and graphene weights not counted); Series 3(denoted by x) is for C-coated Si particles bonded by a resin binderwith specific capacity calculated on the basis of per gram of (Si+C);and Series 4 (denoted by Δ) is for C-coated Si particles hosted by 20%by weight of NGPs with specific capacity calculated on the basis of pergram of (Si+C+NGP).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is related to anode materials for high-capacity lithiumsecondary batteries, which are preferably secondary batteries based on anon-aqueous electrolyte or a polymer gel electrolyte. The shape of alithium secondary battery can be cylindrical, square, button-like, etc.The present invention is not limited to any battery shape orconfiguration.

As an example, a cylindrical battery configuration is shown in FIG. 1. Acylindrical case 10 made of stainless steel has, at the bottom thereof,an insulating body 12. An assembly 14 of electrodes is housed in thecylindrical case 10 such that a strip-like laminate body, comprising apositive electrode 16, a separator 18, and a negative electrode 20stacked in this order, is spirally wound with a separator being disposedat the outermost side of the electrode assembly 14. The cylindrical case10 is filled with an electrolyte. A sheet of insulating paper 22 havingan opening at the center is disposed over the electrode assembly 14placed in the cylindrical case 10. An insulating seal plate 24 ismounted at the upper opening of the cylindrical case 10 and hermeticallyfixed to the cylindrical case 10 by caulking the upper opening portionof the case 10 inwardly. A positive electrode terminal 26 is fitted inthe central opening of the insulating seal plate 24. One end of apositive electrode lead 28 is connected to the positive electrode 16 andthe other end thereof is connected to the positive electrode terminal26. The negative electrode 20 is connected via a negative lead (notshown) to the cylindrical case 10 functioning as a negative terminal.

The positive electrode (cathode) active materials are well-known in theart. The positive electrode 16 can be manufactured by the steps of (a)mixing a positive electrode active material with a conductor agent(conductivity-promoting ingredient) and a binder, (b) dispersing theresultant mixture in a suitable solvent, (c) coating the resultingsuspension on a collector, and (d) removing the solvent from thesuspension to form a thin plate-like electrode. The positive electrodeactive material may be selected from a wide variety of oxides, such asmanganese dioxide, lithium/manganese composite oxide, lithium-containingnickel oxide, lithium-containing cobalt oxide, lithium-containing nickelcobalt oxide, lithium-containing iron oxide and lithium-containingvanadium oxide. Positive electrode active material may also be selectedfrom chalcogen compounds, such as titanium disulfate or molybdenumdisulfate. More preferred are lithium cobalt oxide (e.g., Li_(x)CoO₂where 0.8≦x≦1), lithium nickel oxide (e.g., LiNiO₂), lithium manganeseoxide (e.g., LiMn₂O₄ and LiMnO₂), lithium iron phosphate, lithiumvanadium phosphate because these oxides provide a high cell voltage andgood cycling stability.

Acetylene black, carbon black, ultra-fine graphite particles, or NGPsmay be used as a conductor agent in the cathode. The binder may bechosen from polytetrafluoroethylene (PTFE), polyvinylidene fluoride(PVDF), ethylene-propylene-diene copolymer (EPDM), or styrene-butadienerubber (SBR), for example. Conductive materials such as electronicallyconductive polymers, meso-phase pitch, coal tar pitch, and petroleumpitch may also be used. Preferable mixing ratio of these ingredients maybe 80 to 95% by weight for the positive electrode active material, 3 to20% by weight for the conductor agent, and 2 to 7% by weight for thebinder. The current collector may be selected from aluminum foil,stainless steel foil, and nickel foil. There is no particularlysignificant restriction on the type of current collector, provided thematerial is a good electrical conductor and relatively corrosionresistant. The separator may be selected from a synthetic resin nonwovenfabric, porous polyethylene film, porous polypropylene film, or porousPTFE film.

The negative electrode (anode), which the instant invention provides, isnow explained in detail as follows: The anode composition comprises anelectrochemically active material admixed with nano-scaled grapheneplatelets (NGPs) (e.g. FIG. 2). Both the electrochemically activematerial and the NGPs are capable of absorbing and desorbing lithiumions. For the purpose of defining dimensions of the active material,“micron-sized” in the present context is for those particles having adimension <500 μm or for a coating having a thickness <500 μm. Hence,the electrochemically active material herein discussed is in a finepowder form (having a dimension in the range of 1 nm to 500 μm, moretypically in the range of 10 nm to 200 μm) and/or thin film form(coating, typically in the range of 1 nm to 100 μm, but more typicallyin the range of 1 nm to 1 μm).

An NGP is essentially composed of a sheet of graphene plane or multiplesheets of graphene plane stacked and bonded together through van derWaals forces. Each graphene plane, also referred to as a graphene sheetor basal plane, comprises a two-dimensional hexagonal structure ofcarbon atoms. Each plate has a length and a width parallel to thegraphite plane and a thickness orthogonal to the graphite plane. Bydefinition, the thickness of an NGP is 100 nanometers (nm) or smaller,with a single-sheet NGP being as thin as 0.34 nm. The length and widthof a NGP are typically between 1 μm and 20 μm, but could be longer orshorter. Typically NGPs have a specific surface area greater than about100 m²/gm when they have an average thickness thinner than 10 nm. Thespecific surface area can be greater than about 500 m²/gm when they havean average thickness thinner than 2 nm. Single-sheet graphene has atheoretical specific surface area as high as 2,675 m²/gm.

It is presumed that the top and bottom surfaces of a graphene platelet,single-sheet or multiple-sheet, are capable of adsorbing lithium. Ifsingle-sheet NGPs (single graphene sheets) are dispersed in an amorphouscarbon, there can be a high lithium storage capacity associated with theresulting graphitic carbon morphology, something similar to what wasobserved by Zheng, et al [Ref. 1] and Xue, et al. [Ref. 2]. For amultiple-sheet NGP (e.g., comprising N graphene sheets bonded together),there are (N−1) interstices (spaces between two graphene sheets) thatare capable of accepting or extracting lithium ions via intercalatingand de-intercalating. Since individual or fully separated NGPs do nothave defect areas, such as inter-crystallite boundaries in naturalgraphite or graphitic carbons, NGPs should have a reversible capacityvery close to the theoretical limit (372 mAh/g) for perfectly lithiatedgraphite, LiC₆. With reduced platelet length and width and with all sidesurfaces (edges) of an NGP exposed to the electrolyte, the NGP enablesfast, barrier-free diffusion of Li ions in and out of the interstices.This is conducive to fast charge and discharge of a secondary batterycontaining NGPs as an anode active material.

It may be further noted that the NGPs can be as thin as <0.34 nm (singlegraphene sheet) or 0.68 nm (double-sheet) (e.g., FIG. 2). With a typicallength/width of 1-10 μm, the aspect ratio (length/thickness) can be ashigh as 10 μm/0.34 nm=30,000. Such a unique form factor enables a smallnumber of NGPs dispersed in a matrix material to easily reach apercolation condition to ensure the formation of a network ofelectron-conducting paths. Theoretically, it is possible to use only <1%by volume of NGPs to achieve percolation. However, to have a fullcapability of cushioning the volume change-induced stress or strains ina composite, more than 1% will be best. We recommend at least 5% byweight of NGPs in the composite anode.

The electrochemically active material (e.g., Si particles or film) is incontact with or attached to NGPs. The NGPs can form a network ofelectron transport paths for dramatically improved electricalconductivity or reduced internal resistance (hence, reduced energy lossand internal heat build-up). It appears that the mechanical flexibilityof NGPs enables them to buffer or accommodate the expanded or contractedvolume of the particles or coating during the charge-discharge cyclingof the lithium ion battery, thereby avoiding the fragmentation of theparticles or coating, or loss of contact with the anode currentcollector.

The electrochemically active material in the present invention ispreferably selected from the following groups of materials:

-   -   (1) Silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony        (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), and cadmium (Cd);        preferably of nanocrystalline or amorphous structure in a thin        film (coating) or micron- or nanometer-sized particulate form.        The coating is preferably thinner than 10 μm and more preferably        thinner than 1 μm; This group of material was chosen for our        studies due to the notion that their theoretical capacity is        significantly higher than that of graphite alone: Li_(4-4.4)Si        (3,829-4,200 mAh/g), Li_(4.4)Ge (1,623 mAh/g), Li_(4.4)Sn (993        mAh/g), Li₃Cd (715 mAh/g), Li₃Sb (660 mAh/g), Li_(4.4)Pb (569        mAh/g), LiZn (410 mAh/g), and Li₃Bi (385 mAh/g).    -   (2) The alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb,        Bi, Zn, Al, or Cd, stoichiometric or non-stoichiometric, with        other elements;    -   (3) The oxides, carbides, nitrides, sulfides, phosphides,        selenides, tellurides, pnictides, or their mixtures (e.g.,        co-oxides or composite oxides) of Si, Ge, Sn, Pb, Sb, Bi, Zn,        Al, Fe, or Cd.    -   (4) Salts or hydroxides of Sn, e.g., SnSO₄ (600 mAh/g),        Sn₂PO₄Cl, (300 mAh/g even after 40 cycles), and Sn₃O₂(OH)₂ (300        mAh/g).

The NGPs can be obtained from the intercalation, exfoliation, andseparation of graphene sheets of a laminar graphite material, explainedas follows: Carbon materials can assume an essentially amorphousstructure (glassy carbon), a highly organized crystal (graphite crystalor crystallite), or a whole range of intermediate structures that arecharacterized by having various proportions and sizes of graphitecrystallites and defects dispersed in an amorphous carbon matrix.Typically, a graphite crystallite is composed of a number of graphenesheets or basal planes (also referred to as a-b planes) that are bondedtogether through van der Waals forces in the c-axis direction, thedirection perpendicular to the basal plane. These graphite crystallitesare typically micron- or nanometer-sized in the a- or b-direction (theseare called La dimension). The c-directional dimension (or thickness) iscommonly referred to as Lc. The interplanar spacing of a perfectgraphite is known to be approximately 0.335 nm (3.35 Å). The graphitecrystallites are dispersed in or connected by crystal defects or anamorphous phase in a laminar graphite particle, which can be a graphiteflake (natural or synthetic, such as highly oriented pyrolytic graphite,HOPG), graphite spherule (spheroidal graphite or micro graphite ball),carbon/graphite fiber segment, carbon/graphite whisker, carbon/graphitenano-fiber (CNF or GNF), meso-phase micro-bead (MCMB). In the case of acarbon or graphite fiber segment, the graphene plates may be a part of acharacteristic “turbostratic” structure.

According to a preferred embodiment of the present invention, a laminargraphite material can be subjected to a chemical treatment(intercalation and/or oxidation) to form a graphite intercalationcompound (GIC). The GIC is then exposed a thermal shock (at a hightemperature, typically 800-1,050° C.) for a short period of time(typically 15-60 seconds). The resulting products are networks ofexfoliated graphite flakes commonly referred to as graphite worms. Agraphite worm is characterized as having a network of largelyinterconnected exfoliated flaks with pores between flakes. The flakeshave a typical length/width/diameter dimension of 0.5-100 μm (moretypically 1-20 μm) and typical thickness of 0.34 nm-500 nm (moretypically 10-100 nm). These flakes in a graphite worm remainsubstantially interconnected (physically in contact with each other orbonded to each other). Hence, a mechanical shearing treatment istypically needed to break up the flakes to form separated or isolatedNGPs. It may be noted that individual NGPs are basically graphitecrystals that have a high in-plane electrical conductivity, up to10⁴-10⁵ S/cm, which is orders of magnitude higher than that of carbonblack, activated carbon, polymeric carbon, amorphous carbon, hardcarbon, soft carbon, and meso-phase pitch, etc. This is particularlyuseful since an anode should have a low electrical resistance to reducethe energy loss. Individual NGPs have a density close to 2.3 g/cm³.

In the present invention, upon mixing NGPs with an electrochemicallyactive material, the resulting mixture may be subjected tore-compression to form an integral anode structure, in which the activematerial particles or coating are further held in place between NGPs(with or without a resin binder). Preferably, the NGP amount is in therange of 2% to 90% by weight and the amount of particles or coating isin the range of 98% to 10% by weight. It may be noted that Greinke, etal., in U.S. Pat. No. 6,555,271 (Apr. 29, 2003), disclosed a process forproducing a lithium ion battery anode, the process comprising laminatingparticles of exfoliated graphite to a metallic substrate, such that theparticles of exfoliated graphite form a binder-free sheet of graphite(flexible graphite sheet) having a thickness of no more than about 350microns. However, no other non-carbon based electrochemically activematerial was included in the anode and, hence, the exfoliated graphiteflakes tend to be highly oriented along the sheet plane direction(possibly making a significant amount of graphite flakes unaccessible tolithium ions). Hence, the best achievable reversible capacity remainsmuch lower than 372 mAh/g of flexible graphite anode (the best reportedvalve being 0.51×372 mAh/g).

In one preferred embodiment, the NGPs are derived from natural graphite,synthetic graphite, highly oriented pyrolytic graphite, graphite fiber,carbon fiber, carbon nano-fiber, graphitic nano-fiber, sphericalgraphite or graphite globule, meso-phase micro-bead (MCMB), meso-phasepitch, graphitic coke, or polymeric carbon. For instance, natural flakegraphite may be subjected to an electrochemical or chemicalintercalation treatment under a condition comparable to what has beencommonly employed to prepare the so-called expandable graphite or stablegraphite intercalation compound. This can be accomplished by immersinggraphite powder in a solution of sulfuric acid, nitric acid or nitrate,and potassium permanganate for preferably 1-24 hours (details to bedescribed later). The resulting acid-intercalated graphite compound isthen subjected to washing and rinsing and then dried to obtainexpandable graphite. The expandable graphite is then heat-exfoliated andmechanically sheared to obtain NGPs.

The spheroidal graphite, produced by spheroidizing natural graphiteflakes using a special thermo-chemical procedure, is available fromseveral commercial sources (e.g., Huadong Graphite Co., Pingdu, China).The spheroidal graphite has a basically identical crystalline structureas in natural graphite, having relatively well-ordered crystallites withan interplanar spacing of 0.336 nm. The MCMB is obtained by extractingmeso-phase particles out of other less-ordered carbon matrix and thengraphitizing the meso-phase particles. They are typically supplied as ahighly graphitic form of graphite. Commercial sources of MCMBs includeOsaka Gas Company, Japan, China Steel Chemical Co., Taiwan, and ShanghaiShanshan Technology Co., China.

The starting laminar graphite material preferably has a numericalparticle size (measured by a laser scattering method) that is smallerthan about 25 μm, more preferably smaller than about 10 μm, and mostpreferably smaller than about 5 μm. The smaller particle size, leadingto smaller NGPs, reduces lithium diffusion distances and increases therate capability of the anode, which is a factor in preventing lithiumplating at the anode.

For use as a starting laminar graphite material, meso-phase pitch,graphitic coke, or polymeric carbon may require additionalgraphitization treatment, typically at a temperature in the range of1,500 to 3,000° C. to form nano- or micron-scaled graphite crystallitesdispersed in an amorphous carbon matrix. Such a blend or composite ofgraphitic phase (graphite crystallites) and non-crystalline phase isthen subjected to the same intercalation treatment to obtain anexpandable graphite sample. The expandable graphite is then exposed to athermal shock to obtain exfoliated graphite worms, which are the hostmaterial in the presently invented anode material.

The electrochemically active particles or coating of an anode materialaccording to a preferred embodiment of the invention include at leastone of silicon (Si), germanium (Ge), and tin (Sn) as an element. This isbecause silicon, germanium, and tin have a high capability of insertingand extracting lithium, and can reach a high energy density. The nextpreferred group of elements includes lead (Pb), antimony (Sb), bismuth(Bi), zinc (Zn), aluminum (Al), and cadmium (Cd). When any of these twosets of elements are included as a primary element of anelectrochemically active material (defined as being capable of absorbingand extracting lithium ions in the present context), which is hosted byexfoliated graphite flakes, the cycling stability of the resulting anodematerial can be significantly improved.

Preferred examples of such an active material include the simplesubstance (metal element), an alloy, or a compound of silicon (Si),germanium (Ge), or tin (Sn), and combinations thereof. In general, theactive material may include only one kind or a mixture of a plurality ofkinds selected from the group consisting of Si, Ge, Sn, Pb, Sb, Bi, Zn,Al, and Cd. In the invention, the alloy or compound may include one ormore kinds of metal elements from this group and one or more kinds ofmetal elements from other groups. Further, the alloy or compound mayinclude a non-metal element. The active alloy or compound material maybe a solid solution, a eutectic (eutectic mixture), an intermetalliccompound (stoichiometric or non-stoichiometric), or the coexistence oftwo or more kinds selected from them. Preferably, the material comprisesa nanocrystalline or amorphous phase.

As an alloy or compound of silicon, for example, an active material mayinclude at least one element selected from the group consisting of tin(Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn),zinc (Zn), indium (In), silver (Ag), titanium, germanium (Ge), bismuth(Bi), antimony (Sb) and chromium (Cr) as a second element in addition tosilicon. As an alloy or compound of tin, for example, an active materialmay include at least one kind selected from the group consisting ofsilicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver,titanium, germanium, bismuth, antimony and chromium as a second elementin addition to tin.

As a compound of silicon or a compound of tin, for example, a compoundincluding oxygen (O), carbon (C), nitrogen (N), sulfur (S), orphosphorous (P) may be used, and the compound may include theabove-described second element in addition to tin or silicon. Apreferred example is a SnCoC-containing material in which tin, cobaltand carbon are included as elements, and the carbon content is within arange from 9.9 wt % to 29.7 wt % inclusive, and the ratio Co/(Sn+Co) ofcobalt to the total of tin and cobalt is within a range from 30 wt % to70 wt % inclusive, because a high energy density and superior cyclecharacteristics can be obtained within such a composition range for anelectrochemically active material hosted by exfoliated graphite flakes.

The SnCoC-containing material may further include any other element, ifnecessary. As the element, for example, silicon, iron, nickel, chromium,indium, niobium (Nb), germanium, titanium, molybdenum (Mo), aluminum,phosphorus (P), gallium (Ga) or bismuth is preferable, and two or morekinds selected from them may be included. This suggestion is based onthe observation that the capacity and the cycle characteristics ofanodes can be further improved. The SnCoC-containing material includes aphase including tin, cobalt and carbon, and the phase preferably has anano-crystalline structure or an amorphous structure. Moreover, in theSnCoC-containing material, at least a part of carbon if added as anelement, is preferably bonded to a metal element or a metal compound.This is based on the consideration that a decline in the cyclecharacteristics of prior art lithium ion battery is caused by cohesionor crystallization of tin or the like. When carbon is bonded to themetal or compound, such cohesion or crystallization can be inhibited.However, we have observed that, with the active material hosted in thepores between exfoliated graphite flakes, there has been minimal or notcohesion or crystallization. Presumably, this is one of the majoradvantages of using exfoliated graphite flakes as a host.

Such an electrochemically active material can be manufactured, forexample, by simply mixing the materials of all elements to form amixture, melting the mixture in an electric furnace, a high-frequencyinduction furnace, an arc furnace or the like, and then solidifying themixture. The material may also be made by various atomization methods,such as gas atomization or water atomization, various roll methods, ormethods using a mechanochemical reaction, such as a mechanical alloyingmethod or a mechanical milling method. The active material is preferablymanufactured by the method using a mechanochemical reaction, because theactive material can have a low crystalline (or nano-crystalline)structure or an amorphous structure. In this method, for example, amanufacturing apparatus such as a high energy planetary ball mill or amechanical attritor can be used. The resulting fine particles are thenmechanically blended with graphite worms in a dry blender or asolution/liquid mixer. In the later case, the liquid phase involved hasto be removed.

The active material in a thin film or coating form (on a surface ofgraphene platelets) may be formed through depositing the material by,for example, a liquid-phase deposition method, an electrodepositionmethod, a dip coating method, an evaporation method, a sputteringmethod, a CVD (Chemical Vapor Deposition) method, or the like. Thecoating is preferably formed by the liquid-phase deposition method amongthem, because the deposition of an extremely small amount of the activematerial (e.g., SnO or SnO₂) can be easily controlled.

As an example, the liquid-phase deposition method is a method ofdepositing the metal oxide coating on a surface of graphite flakes. Forinstance, this can be accomplished by adding a dissolved species, whicheasily coordinates fluorine (F) as an anion trapping agent, into a metalfluoride complex solution, and immersing the exfoliated graphite flakesin the solution, and then trapping a fluorine anion generated from themetal fluoride complex by the dissolved species. Alternatively, forexample, a metal compound generating another anion such as a sulfate ionmay be used. The sol-gel technique may be used to impregnate graphiteworms and then subjected to final conversion to the fine particles, suchas silicon dioxide, SiO₂.

Another preferred class of electrochemically active material that can behosted by NGPs includes the oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or their mixtures (e.g., co-oxides orcomposite oxides) of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, or Cd. They canbe readily produced in a fine powder or thin film form. For instance,SnO or SnO₂ may be admixed with oxides of B, Al, P, Si, Ge, Ti, Mn, Fe,or Zn and then subjected to heat treatments to obtain composite oxides.Composite oxides may also be prepared by mechanical alloying (e.g., ballmilling of a mixture of SnO and B₂O₃). SnO or SnO₂ alone is ofparticular interest due to their high theoretical capacities. Ironoxides or phosphates are of interest since Li₆Fe₂O₃ has a theoreticalcapacity of 1,000 mAh/g. The first cycle capacity of Fe₃PO₇ is found toreach 800 mAh/g. The capacity of SnS₂ is as high as 620 mAh/g and isstable under charge-discharge cycling conditions.

Combined atomization (or vaporization) and reaction can be used toobtain the oxides, carbides, nitrides, sulfides, phosphides, selenides,tellurides, or their mixtures, as illustrated in W. C. Huang, “Methodfor the Production of Semiconductor Quantum Particles,” U.S. Pat. No.6,623,559 (Sep. 23, 2003) and J. H. Liu and B. Z. Jang, “Process andApparatus for the Production of Nano-Scaled Powders,” U.S. Pat. No.6,398,125 (Jun. 4, 2002).

It may be noted that Li₃N has a high lithium ion conductivity. Whenallowed to react with a transition metal M, such as Co, Ni, or Cu, theLi₃N can be readily transformed into Li_(3-x)M_(x)N. During the lithiumion extraction process, the nitride is transformed from a crystallinestate to an amorphous state, resulting in a significant volume change.Due to the presence of NGPs, such a volume change did not lead to avolume change-induced failure of the anode and the anode appeared toprovide a much better cycling response. As an example, we observed thatthe reversible capacity of Li_(3-x)Cu_(x)N and that of Li_(3-x)Co_(x)N(both in a 25% by weight of NGPs) reaches 720 mAh/g and 610 mAh/g,respectively. In contrast, the reversible capacities of the twonitrides, when used alone (but bonded with 5% PVDF binder), wereapproximately 650 mAh/g and 560 mAh/g, respectively. It is of interestto note again that the theoretical capacity of an ideal graphitematerial is 372 mAh/g and the actual reversible capacities of most ofthe graphite materials are much lower than 320 mAh/g. This observationdemonstrates that a combination of a lithium-transition metal nitrideand NGPs provides an unexpected, synergistic effect on the reversiblecapacity of a lithium ion battery.

An amorphous carbon phase may be added to the active material-NGPcomposite material in the following manner: The composite may be mixedwith a resin to form a composite mixture. This composite mixture may beheated to a temperature of typically 500-1,000° C. for a sufficientperiod of time to convert the resin into a polymeric carbon or anamorphous carbon phase. Hence, in the presently invented negativeelectrode material composition, the composite material may furthercomprise an amorphous carbon phase or polymeric carbon, wherein theelectrochemically active particles or coating may be bonded by anamorphous carbon phase or polymeric carbon. Alternatively, the amorphouscarbon phase may be obtained from chemical vapor deposition (CVD),chemical vapor infiltration (CVI), or pyrolyzation of an organicprecursor. CVD or CVI techniques are well-known in the art and have beenutilized to cover a graphite material with an amorphous coating.

Further alternatively, the electrochemically active material, if in finepowder form, may be further bonded by a binder material. An electricallyconductive binder material may be selected from coal tar pitch,petroleum pitch, meso-phase pitch, coke, a pyrolized version of pitch orcoke, or a conjugate chain polymer (intrinsically conductive polymersuch as polythiophene, polypyrrole, or polyaniline. Alternatively, theparticles may be bonded by a non-conductive material, such aspolyvinylidene fluoride (PVDF) or PTFE, to form an integral anodemember.

A wide range of electrolytes can be used for practicing the instantinvention. Most preferred are non-aqueous and polymer gel electrolytesalthough other types can be used. The non-aqueous electrolyte to beemployed herein may be produced by dissolving an electrolytic salt in anon-aqueous solvent. Any known non-aqueous solvent which has beenemployed as a solvent for a lithium secondary battery can be employed. Anon-aqueous solvent mainly consisting of a mixed solvent comprisingethylene carbonate (EC) and at least one kind of non-aqueous solventwhose melting point is lower than that of aforementioned ethylenecarbonate and whose donor number is 18 or less (hereinafter referred toas a second solvent) may be preferably employed. This non-aqueoussolvent is advantageous in that it is (a) stable against a negativeelectrode containing a carbonaceous material well developed in graphitestructure; (b) effective in suppressing the reductive or oxidativedecomposition of electrolyte; and (c) high in conductivity. Anon-aqueous electrolyte solely composed of ethylene carbonate (EC) isadvantageous in that it is relatively stable against decompositionthrough a reduction by a graphitized carbonaceous material. However, themelting point of EC is relatively high, 39 to 40° C., and the viscositythereof is relatively high, so that the conductivity thereof is low,thus making EC alone unsuited for use as a secondary battery electrolyteto be operated at room temperature or lower. The second solvent to beused in a mixture with EC functions to make the viscosity of the solventmixture lower than that of EC alone, thereby promoting the ionconductivity of the mixed solvent. Furthermore, when the second solventhaving a donor number of 18 or less (the donor number of ethylenecarbonate is 16.4) is employed, the aforementioned ethylene carbonatecan be easily and selectively solvated with lithium ion, so that thereduction reaction of the second solvent with the carbonaceous materialwell developed in graphitization is assumed to be suppressed. Further,when the donor number of the second solvent is controlled to not morethan 18, the oxidative decomposition potential to the lithium electrodecan be easily increased to 4 V or more, so that it is possible tomanufacture a lithium secondary battery of high voltage.

Preferable second solvents are dimethyl carbonate (DMC), methylethylcarbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methylpropionate, propylene carbonate (PC), .ganmma.-butyrolactone(.gamma.-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate(PF), methyl formate (MF), toluene, xylene and methyl acetate (MA).These second solvents may be employed singly or in a combination of twoor more. More desirably, this second solvent should be selected fromthose having a donor number of 16.5 or less. The viscosity of thissecond solvent should preferably be 28 cps (centipoises) or less at 25°C.

The mixing ratio of the aforementioned ethylene carbonate in the mixedsolvent should preferably be 10 to 80% by volume. If the mixing ratio ofthe ethylene carbonate falls outside this range, the conductivity of thesolvent may be lowered or the solvent tends to be more easilydecomposed, thereby deteriorating the charge/discharge efficiency. Morepreferable mixing ratio of the ethylene carbonate is 20 to 75% byvolume. When the mixing ratio of ethylene carbonate in a non-aqueoussolvent is increased to 20% by volume or more, the solvating effect ofethylene carbonate to lithium ions will be facilitated and the solventdecomposition-inhibiting effect thereof can be improved.

Examples of preferred mixed solvent are a composition comprising EC andMEC; comprising EC, PC and MEC; comprising EC, MEC and DEC; comprisingEC, MEC and DMC; and comprising EC, MEC, PC and DEC; with the volumeratio of MEC being controlled within the range of 30 to 80%. Byselecting the volume ratio of MEC from the range of 30 to 80%, morepreferably 40 to 70%, the conductivity of the solvent can be improved.With the purpose of suppressing the decomposition reaction of thesolvent, an electrolyte having carbon dioxide dissolved therein may beemployed, thereby effectively improving both the capacity and cycle lifeof the battery.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃) and bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂]. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ arepreferred. The content of aforementioned electrolytic salts in thenon-aqueous solvent is preferably 0.5 to 2.0 mol/l.

EXAMPLES

In the examples discussed below, unless otherwise noted, raw materialssuch as silicon, germanium, bismuth, antimony, zinc, iron, nickel,titanium, cobalt, and tin were obtained from either Alfa Aesar of WardHill, Mass., Aldrich Chemical Company of Milwaukee, Wis. or Alcan MetalPowders of Berkeley, Calif. X-ray diffraction patterns were collectedusing a Siemens diffractometer equipped with a copper target x-ray tubeand a diffracted beam monochromator. The presence or absence ofcharacteristic patterns of peaks was observed for each of the alloysamples studied. For example, a phase was considered to be amorphouswhen the X-ray diffraction pattern was absent or lacked sharp,well-defined peaks. The grain sizes of the crystalline phases weredetermined by the Scherer equation. When the grain size was calculatedto be less than 50 nanometers, the phase was considered to benanocrystalline. In several cases, scanning electron microscopy (SEM)and transmission electron microscopy (TEM) were used to characterize thestructure and morphology of the hybrid material samples.

In Comparative Examples 1-7, each of the electrochemically activematerials alone (without being hosted by NGPs) was formed into anelectrode and characterized in an electrochemical cell. In addition, inExamples 1-7, the corresponding composite material was also separatelyincorporated into a lithium ion battery as an anode. Specifically, thepowder particles obtained in the Comparative Samples were separatelymixed with, as a binder, 2.2% by weight of styrene/butadiene rubber and1.1% by weight of carboxylmethyl cellulose to obtain a mixture (aprecursor to an anode active material), which was then coated on acopper foil to be employed as a collector. After being dried, thepowder/resin mixture-copper foil configuration was hot-pressed to obtaina negative electrode. In some cases, PVDF particles were used as abinder material. Powder particles may also be bonded by anelectronically conductive polymer. For instance, polyaniline-maleicacid-dodecyl hydrogensulfate salt may be synthesized directly viaemulsion polymerization pathway using benzoyl peroxide oxidant, sodiumdodecyl sulfate surfactant, and maleic acid as dopants. Drypolyaniline-based powder may be dissolved in DMF up to 2% w/v to form asolution.

Unless otherwise noted, the cathode of a lithium ion battery wasprepared in the following way. First, 91% by weight of lithium cobaltoxide powder LiCoO₂, 3.5% by weight of acetylene black, 3.5% by weightof graphite, and 2% by weight of ethylene-propylene-diene monomer powderwere mixed together with toluene to obtain a mixture. The mixture wasthen coated on an aluminum foil (30 μm) serving as a current collector.The resulting two-layer aluminum foil-active material configuration wasthen hot-pressed to obtain a positive electrode.

A positive electrode, a separator composed of a porous polyethylenefilm, and a negative electrode was stacked in this order. The stackedbody was spirally wound with a separator layer being disposed at theoutermost side to obtain an electrode assembly as schematically shown inFIG. 1. Hexafluorolithium phosphate (LiPF₆) was dissolved in a mixedsolvent consisting of ethylene carbonate (EC) and methylethyl carbonate(MEC) (volume ratio: 50:50) to obtain a non-aqueous electrolyte, theconcentration of LiPF₆ being 1.0 mol/l (solvent). The electrode assemblyand the non-aqueous electrolyte were placed in a bottomed cylindricalcase made of stainless steel, thereby obtaining a cylindrical lithiumsecondary battery.

Unless otherwise specified, in order to compare the electrochemicalbehaviors of composite anode materials prepared in Examples 1-7, we useda method analogous to that used by Wu, et al. [Y. P. Wu, C. Jiang, C.Wan and E. Tsuchida, “A Green Method for the Preparation of AnodeMaterials for Lithium Ion Batteries,” J. Materials Chem., 11 (2001)1233-1236] to characterize the charge and discharge behaviors ofcorresponding lithium ion batteries.

Example 1 Samples 1 a, 1 b and Comparative Samples 1 a, 1 b

Natural flake graphite, nominally sized at 45 μm, provided by AsburyCarbons (405 Old Main St., Asbury, N.J. 08802, USA) was milled to reducethe size to approximately 14 μm (Sample 1). The chemicals used in thepresent study, including fuming nitric acid (>90%), sulfuric acid(95-98%), potassium chlorate (98%), and hydrochloric acid (37%), werepurchased from Sigma-Aldrich and used as received.

A reaction flask containing a magnetic stir bar was charged withsulfuric acid (360 mL) and nitric acid (180 mL) and cooled by immersionin an ice bath. The acid mixture was stirred and allowed to cool for 15min, and graphite (20 g) was added under vigorous stirring to avoidagglomeration. After the graphite powder was well dispersed, potassiumchlorate (110 g) was added slowly over 15 min to avoid sudden increasesin temperature. The reaction flask was loosely capped to allow evolutionof gas from the reaction mixture, which was stirred for 48 hours at roomtemperature. On completion of the reaction, the mixture was poured into8 L of deionized water and filtered. The slurry was spray-dried torecover an expandable graphite sample. The dried, expandable graphitewas quickly placed in a tube furnace preheated to 1,000° C. and allowedto stay inside a quartz tube for approximately 40 seconds to obtainexfoliated graphite worms. The worms were then subjected to anultrasonication treatment (80 W for one hour) for flake break-up andseparation of graphene sheets to obtain NGPs. A Branson S450Ultrasonicator was used.

Sample 1 a: Approximately 5 grams of the NGPs thus prepared were mixedwith 25 grams of SnCl₂ in a glass flask under nitrogen environment. Themixture was heated at 370° C. for approximately 10 hours. The productwas rinsed in distilled water for one minute and then dried in ambientair for one hour. The dried product was post-heated at 1,000° C. for onehour, resulting in a product that is composed of approximately 55% byweight Sn nanoparticles disposed in pores of the graphite worms.

Comparative Sample 1 a: Sn nano particles were prepared in a similarmanner without the presence of NGPs. These particles were bonded by abinder material (2.2% by weight of styrenelbutadiene rubber and 1.1% byweight of carboxylmethyl cellulose) in the preparation of an anodemember. NGPs with a binder material but without the Sn particles, werealso made by re-compression into a baseline anode material.

The reversible capacities of Sample 1 a, Comparative Sample 1 a, abaseline NGP anode sample, and a theoretically predicted model (based ona rule-of-mixture law) are shown in FIG. 3. The prediction was based onthe assumption of no synergistic effect between the twoelectrochemically active materials (Sn and NGPs) according to thefollowing equation: C_(hybrid)=f_(Sn)C_(Sn)+f_(NGP)C_(NGP), whereC_(hybrid) is the predicted specific capacity of the hybrid material,f_(Sn) and f_(NGP) are the weight fractions of the Sn particles and theNGP, respectively, and C_(Sn) and C_(NGP) are the specific capacities ofthe Sn particles alone and the NGPs alone, respectively. It is clearthat the Sn particles, when hosted by the NGPs, provide a synergisticeffect on the specific capacity, which is far superior to what could beachieved by either component alone. This is a highly surprising andimpressive result.

Sample 1 b: About 5 grams of Si powder were put in a tungsten heatingboat. Approximately 5 grams of NGPs supported by a quartz plate of 30cm×5 cm and the Si-loaded tungsten boat were mounted in a vacuumchamber, which was evacuated to and maintained at a pressure of 10⁻⁷torr for 3 hours at room temperature. An electric current was passeddirectly on the tungsten boat to heat the loaded Si up to slightly aboveits melting point. The evaporation was controlled by monitoring thedeposited thickness with a quartz crystal microbalance mounted near thegraphite worm plate. The deposition rate was controlled to be about 2Å/min. The resulting product was a hybrid material containing a Si thinfilm coating on NGPs. Weight measurements before and after Si vapordeposition indicated that the composite material was composed of 23% byweight Si and 77% by weight NGPs.

Comparative Sample 1 b: The Si thin film was coated onto a surface of a30 μm thick Ni foil and the resulting Si-coated Ni foil was used as ananode in a lithium ion battery. The Si-coated foil was cut into 1 cm×1cm squares with a Ni lead wire spot-welded thereon. The electrolyte usedwas 1 M LiClO₄ dissolved in propylene carbonate (PC). A Pyrexcylindrical cell with three electrodes was used to evaluateconstant-current charge-discharge cycling behaviors, wherein puremetallic Li foils were used as the reference and counter electrodes.

The data in FIG. 4 show that a dramatic loss in specific capacityoccurred to the Ni-supported Si film sample. However, even after 1000cycles, the NGP-supported Si film still maintains an exceptionally highspecific capacity. In both cases, the specific capacity was calculatedbased on the Si weight only; the host or support weight was not counted.SEM examinations indicate that the Si film supported on Ni foilexhibited fragmentation and delamination (away from the support layer)after the first cycle. This degradation phenomenon was not observed withthe NGP-supported Si film.

Example 2 Samples 2 a-2 e and Comparative Samples 2 a-2 e

NGPs for Samples 2 a, 2 b, 2 c, 2 d, and 2 e were prepared according tothe same procedure used for Sample 1, but the starting graphitematerials were highly oriented pyrolytic graphite (HOPG), graphite fiber(Amoco P-100 graphitized carbon fiber), graphitic carbon nano-fiber(Pyrograph-III from Applied Science, Inc., Cedaville, Ohio), spheroidalgraphite (HuaDong Graphite, QinDao, China), and meso-carbon micro-beads(MCMBs) (MCMB 2528, Osaka Gas Company, Japan), respectively. These fourtypes of laminar graphite materials were intercalated, exfoliated, andseparated to obtain NGPs under similar conditions as in Sample 1.

The corresponding electrochemically active particles for these samplesinclude materials derived from metal oxides of the type MO or MO₂, whereM=Sn, Pb, Ge, Si, or Cd. The active materials may be prepared accordingto the following steps: (1) stanous oxide (SnO) powder and lithiumnitride (Li₃N) powder were mixed in a stoichiometric ratio of two molesof Li₃N to three moles of SnO; (2) the mixture of powders from step (1)was fed into a planetary ball mill (Model PM-400 from Glen Mills,Clifton, N.J.), and the milling was proceeded until the SnO and Li₃Nreached a state characterized by complete disappearance of the X-raydiffraction patterns for crystalline SnO and Li₃N, and the subsequentappearance of the X-ray patterns for amorphous Li₂O and crystalline Sn.The ball milling process typically lasts for one to two days at ambienttemperature; and (3) The milled powder was then blended with NGPsobtained in Sample 2 a (i.e., from the HOPG) and the resulting powdermixture was press-rolled to form an anode layer of approximately 200 μm.The layer is composed of approximately 72% by weight of the Li₂O—Snmixture and 72% by weight of NGPs. The Comparative Sample 2 a wasprepared from the Li₂O—Sn mixture powder bonded with approximately 5%resin binder.

Samples 2 b-2 e were obtained from Pb, Ge, Si, and Cd, respectively asan electrochemically active material used in combination with NGPs fromthe graphite fiber, graphitic carbon nano-fiber (GNF), spheroidalgraphite (SG), and MCMB, respectively.

Comparative Samples 2 a-2 e were obtained from similarly made powdermixtures, which were not hosted by NGPs, but instead only bonded by aresin binder in the preparation of an anode.

The specific capacity data for Sample 2 a and Comparative Sample 2 a,summarized in FIG. 5, clearly show that the NGP-hosted mixture of Sn andLi₂O has a better cycling response than the corresponding resinbinder-bonded material. By defining the extent ofirreversibility=(initial discharge capacity−discharge capacity after Ncycles)/(initial discharge capacity), Sample 2 a and Comparative sample2 a exhibit an extent of capacity (N=20) of 4.3% and 33.3%,respectively. The extents of irreversibility of Samples 2 b, 2 c, 2 d,and 2 e are 5.8%, 5.1%, 6.1%, and 4.6%, respectively. In contrast, theextents of irreversibility of Comparative Samples 2 b, 2 c, 2 d, and 2 eare 25.6%, 26.2%, 27.3%, and 25.6%, respectively.

Example 3 Samples 3 a-3 c and Comparative Samples 3 a-3 c

Additional graphite intercalation compound (GIC) was prepared byintercalation and oxidation of natural graphite flakes (original size of200 mesh, from Huadong Graphite Co., Pingdu, China, milled toapproximately 15 μm, referred to as Sample 3 a) with sulfuric acid,sodium nitrate, and potassium permanganate according to the method ofHummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. In this example, forevery 1 gram of graphite, we used a mixture of 22 ml of concentratedsulfuric acid, 2.8 grams of potassium permanganate, and 0.5 grams ofsodium nitrate. The graphite flakes were immersed in the mixturesolution and the reaction time was approximately one hour at 30° C. Itis important to caution that potassium permanganate should be graduallyadded to sulfuric acid in a well-controlled manner to avoid overheat andother safety issues. Upon completion of the reaction, the mixture waspoured into deionized water and filtered. The sample was then washedrepeatedly with deionized water until the pH of the filtrate wasapproximately 5. The slurry was spray-dried and stored in a vacuum ovenat 60° C. for 24 hours. The resulting GIC was exposed to a temperatureof 1,050° C. for 35 seconds in a quartz tube filled with nitrogen gas toobtain worms of exfoliated graphite flakes. The worms were thensubjected to an ultrasonication treatment (85 W for one hour) for flakebreak-up and separation of graphene sheets to obtain NGPs.

The electrochemically active particles in these examples areSi_(x)Sn_(q)M_(y)C_(z) type compositions with (q+x)>(2 y+z), x>0, and Mis one or more metals selected from manganese, molybdenum, niobium,tungsten, tantalum, iron, copper, titanium, vanadium, chromium, nickel,cobalt, zirconium, yttrium, or combinations thereof, wherein the Si, Sn,M, and C elements are arranged in the form of a multi-phasemicrostructure comprising at least one amorphous or nanocrystallinephase. As an example, Sample 3 a was prepared by ball-milling siliconchips, cobalt powder, and graphite powder with 28 tungsten carbide balls( 5/16-inches each, approximately 108 grams) for 4 hours in a 45milliliter tungsten carbide vessel using a planetary ball mill (ModelPM-400 from Glen Mills, Clifton, N.J.) under an argon atmosphere. Thevessel was then opened, chunks of caked powder were broken up, and themilling was continued for an additional hour in an argon atmosphere. Thetemperature of the tungsten carbide vessel was maintained at about 30°C. by air cooling. The product was determined to be approximatelySi₇₃CO₂₃C₄.

Samples 3 b and 3 c were prepared by the same general procedure asSample 3 a with the following differences. Silicon powder (325 mesh),tin powder (<10 microns), and Co or Ni metal powders (Cobalt, 1.6microns; Nickel, Alcan Metal Powder, Type 123) were used. Samples 3 band 3 c were milled with 14 tungsten carbide balls (0.415 mm diameter;approximately 54 grams total weight) in the ball mill for 16 hours in anargon atmosphere. The resulting products were Si₇₄Sn₂CO₂₄ andSi₇₃Sn₂Ni₂₅, respectively.

For Samples 3 a-3 c, the fine powder products prepared after ballmilling were separately mixed with NGPs, plus 2% by weight of a resinbinder, and then cold-compressed with a hydraulic press at 100 psig toprepare an anode sheet of approximately 200 μm in thickness. ForComparative Samples 3 a-3 c, the fine powder products were not hosted byNGPs, but instead only bonded by a resin binder in the preparation of ananode. The reversible capacities of Samples 3 a, 3 b, and 3 c after 40cycles were 1,160, 1222, 1213 mAh/g, respectively. In contrast, thereversible capacities of Comparative Samples 3 a, 3 b, and 3 c afteronly 10 cycles were 782, 740, 892 mAh/g, respectively. Theses classes ofmaterials were characterized as having primarily amorphous andnanocrystalline phases, which were believed to be less prone to volumechange-induced particle fragmentation or loss of particle-to-particle orparticle-to-current collector contact. Our data have clearly shown thateven these types of electrochemically active materials can be furtherprotected by the NGP host.

Example 4 Sample 4 a, 4 b and Comparative Samples 4 a, 4 b

Five grams of NGPs prepared in Example 3 were used in the preparation ofSample 4 a, 4 b. The electrochemically active powders were prepared bymixing Li₃N with stoichiometric amounts of a transition metal, such asCu and Co, respectively. The powder mixtures were separately sealed incontainers of a planetary ball mill, which was operated for two days toobtain nitrides, such as Li_(3-x)Cu_(x)N and Li_(3-x)Co_(x)N. Weobserved that the reversible capacity of Li_(3-x)Cu_(x)N (e.g. x=0.4)and that of Li_(3-x)Co_(x)N (e.g., x=0.3), both hosted by a 25% byweight of NGPs, reached 732 mAh/g and 612 mAh/g, respectively when thecell was operated under a constant current density of 0.5 mA/cm² in thevoltage range of 0-1.4 V, using Li metal as a reference electrode. Incontrast, the reversible capacities of the two nitrides, when used alone(but bonded with 5% PVDF binder), were approximately 650 mAh/g and 560mAh/g, respectively.

Example 5 Samples 5 a and Comparative Samples 5 a

The same batch of NGPs prepared in Example 3 was used in Example 5.

Sample 5 a: Nano-crystalline Si with an average particle size of 80 nmwas produced in our laboratory using a twin-arc evaporation technique.Carbon-coated Si was prepared by a thermal vapor decomposition (TVD)technique. The nano powder was supported on a quartz plate positionedinside a reaction tube. Benzene vapor and nitrogen gas were fed into thereaction tube (1,000° C.) at flow rates of 2 mL/min and 1 L/min,respectively. At such a high temperature, the organic vapor decomposedand carbon deposited onto the surface of Si particles. The mean particlesize of the carbon-coated silicon was found to be approximately 84 nm(approximately 13% by weight carbon). Carbon-coated Si particles weredry-blended with to produce a composite material designated as Sample 5a. The corresponding anode material bonded by a resin binder, withoutthe presence of the NGP, is designated as Comparative Sample 5 a.

It may be noted that carbon-coated Si has been investigated as apotential lithium ion battery material, for instance, by a researchgroup at Kyoto University: T. Umeno, et al., “Novel Anode Materials forLithium-Ion Batteries: Carbon-Coated Silicon Prepared by Thermal VaporDecomposition,” Chemistry Letters (2001) 1186-1187 and M. Yoshio, etal., “Carbon-Coated Si as a Lithium-Ion Battery Anode Material,” Journalof the Electrochemical Society, 149 (12) (2002) A1598-A1603. They haveshown “the excellent electrochemical performance of carbon-coated Si asanode materials for lithium-ion batteries in terms of high reversiblecapacity over 800 mAh/g, high coulombic efficiency, good cyclability,satisfactory compatibility with both the EC and PC-based electrolytes,and better thermal stability than that of graphite, etc.” They believedthat carbon-coating in the outer layer played a very important role inthe improvement of the electrochemical behavior by not only suppressingthe decomposition of electrolytes on the surface of Si-based electrodes,but also providing integral and continuous electric contact networksaround Si particles even they are a little expanded after lithiuminsertion. However, these researchers have indicated that “If thelithiation capacity of carbon-coated Si is controlled under 1,200 mAh/g,satisfactory cycleability can be obtained.” [T. Umeno, et al. 2001]

By increasing the lithiation capacity to approximately 3,100 mAh/g (pergram of Si, corresponding to an alloy of Li_(3.25)Si), we have foundthat the cycleability of even carbon-coated Si particles was notsatisfactory likely due to a large volume change. However, by addingapproximately 20% by weight of NGPs, we were able to significantlyimprove the cycleability. Shown in FIG. 6 are four data series onspecific capacity: Series 1 (denoted by ♦) is for C-coated Si particlesbonded by a resin binder with specific capacity calculated on the basisof per gram of Si (carbon weight not counted); Series 2 (denoted by ▪)is for C-coated Si particles hosted by 20% by weight of NGPs withspecific capacity calculated on the basis of per gram of Si (carbon andNGP weights not counted); Series 3 (denoted by x) is for C-coated Siparticles bonded by a resin binder with specific capacity calculated onthe basis of per gram of (Si+C); and Series 4 (denoted by Δ) is forC-coated Si particles hosted by 20% by weight of NGPs with specificcapacity calculated on the basis of per gram of (Si+C+NGP). A comparisonbetween Series 1 and Series 2 data indicates that the NGP host enablesthe carbon-coated Si particles to provide a much better cyclingperformance. Furthermore, a comparison between Series 3 and Series 4data indicates that, even after the additional NGP weight is taken intoconsideration, the C-coated Si hosted by the NGP provides a higherabsolute specific capacity than the carbon-coated Si particles withoutan NGP host after 7 charge-discharge cycles.

Example 6 Samples 6 a-6 f and Comparative Samples 6 a-6 f

The same batch of NGPs prepared in Example 3 was used in Example 6.

Sample 6 a: Nano-SnSb alloy was deposited on the surfaces of (NGPs+CNFs)at a weight ratio of 3:7. Between NGPs and CNFs, we had 80% by weight ofNGP particles +20% by weight of CNF (Applied Sciences, Inc.). CNFs wereadded to impart additional mechanical integraity to the anode (withoutadding a resin binder). In brief, SbCl₃ and SnCl₂.H₂O (99%) were mixedtogether with a molar ratio of 5:4 and dissolved in glycerin to form a0.5 M solution. Then, NGP+CNF particles were added to the solution. Themixed solution or suspension was cooled down to 0.0 to 1.0° C. Zn powder(99.9%) with 95% stoichiometric amount was added to the solutiongradually and stirred simultaneously. Finally, after washing by ethanoland filtering, the product was dried under vacuum at 55° C.

For Comparative Samples 6 a, nano-SnSb particles, without NGPs/CNFs,were prepared in a similar manner. The fine powder products were nothosted by NGPs, but instead just bonded by a resin binder in thepreparation of an anode. The extent of irreversibility of Samples 6 awas found to be 5.2% and, in contrast, the extent of irreversibility ofComparative Samples 6 a was 21.5% after 100 cycles.

Sample 6 b: Sn nano particles were prepared by a reduction method inglycerin solution at low temperature, SbCl₃ was dissolved in glycerin toform a 0.5 M solution. Then, NGPs were added to the solution to form asuspension. The suspension was cooled to 0.0 to 1.0° C. Subsequently, Znpowder with a 95% stoichiometric amount was slowly added to the solutionwhile being stirred. Finally, after washing with ethanol and filtering,the black product was dried under vacuum at 50° C. The product was acomposite material containing Sn nano particles well mixed with NGPs,which were then compressed.

For Comparative Samples 6 b, nano-SnSb particles, without NGPs, wereprepared in a similar manner. The fine powder products were not hostedby NGPs, but instead just bonded by a resin binder in the preparation ofan anode. The extent of irreversibility of Samples 6 a was found to be5.5% and, in contrast, the extent of irreversibility of ComparativeSamples 6 a was 25.5% after 100 cycles.

Sample 6 c: The Ni—Sn—P/graphene composite material was prepared by anelectroless plating method in an aqueous solution. First, SnSO₄ (99%),NiSO₄.6H₂O (98%), and NaH₂PO₂. H₂O (95%) in stoichiometric amounts weredissolved in an aqueous solution as the metal precursors after stirringat room temperature. NGP particles were then rapidly added to themetal-precursor solution, and the solution was under continuous stirringat 80° C. for 40 min. A sodium succinate solution was applied as thebuffer to adjust the pH value. The products were washed with distilledwater and filtered until the pH of the filtrate was identical to that ofthe distilled water. The products were then dried at room temperature invacuum. The NGP content was approximately 33% by weight for Sample 6 c.To understand the effect of NGP content, additional samples (ComparativeSamples 6 c-I to 6 c-III) were prepared that contained approximately10%, 5%, and 2% by weight, respectively. The extents of irreversibilityof Samples 6 c, and Comparative Samples 6 c-I, 6 c-II, and 6 c-III werefound to be 4.9%, 4.8%, 4.9%, and 21.3%, respectively. This implies thata minimum NGP amount of 2% (preferably at least 5%) is required toachieve an effective cushioning or protective effect.

Sample 6 d: The starting materials for preparation of SnS nanoparticlesincluded tin (II) chloride (SnCl₂.2H₂O), sodium sulfide hydrate (Na₂S9H₂O), and ethylene glycol (C₂H₆O₂). In a typical procedure, 1.07 gsodium sulfide hydrate and 1.0 g tin (II) chloride were separatelydissolved in an adequate amount of ethylene glycol by magnetic stirring.Then the tin (II) chloride solution was added into the sodium sulfidehydrate solution drop by drop with slow stirring. Upon dropping, thesolution gradually turned from translucent to dark, indicating theformation of SnS particles. All operations above were, conducted athigher than 60° C. NGP particles were then added to the reactingsolution. After complete mixing, the obtained solution was thermostatedat 150° C. for 24 h. Finally, brown black SnS/NGP particles werecollected by centrifugation, washed with de-ionized water and dried at80° C. for 1 day.

For Comparative Samples 6 d, nano-SnS particles, without NGPs, wereprepared in a similar manner. The fine powder products were not hostedby NGPs, but instead just bonded by a resin binder in the preparation ofan anode. The extent of irreversibility of Samples 6 d was found to be5.6 and, in contrast, the extent of irreversibility of ComparativeSamples 6 d was 22.5% after 100 cycles.

Sample 6 e: For the synthesis of carbon aerogel, resorcinol (C₆H₆O₂),formaldehyde (CH₆O) and NH₃H₂O were used. Resorcinol and formaldehydewith a mole ratio of 1:2 were put into de-ionized water and agitated.Then, NH₁₃H₂O was added to the resulting solution to adjust the pH valueto 6.5. Afterwards, the resorcinol-formaldehyde solution was placed inthermostated container at 85° C. When the solution became viscous andappeared orange-like in color, the as-prepared SnS particles (same as inSample 6 d) were mixed with the resorcinol-formaldehyde sol and magneticstirred for several minutes. Then, the viscous sol was mixed with NGPsand the resulting fluid was placed in a thermostated container at 85° C.for 24 h to obtain SnS dispersed resorcinol-formaldehyde gel film coatedon graphene platelet surfaces. The synthesized gel film was carbonizedat 650° C. for 1.5 h under nitrogen. The content of SnS in the SnS/Ccomposite film is 72 wt %, determined from the weight loss before andafter carbonization. Comparative Sample 6 e was similarly preparedwithout using NGPs. The extent of irreversibility of Samples 6 a wasfound to be 64 after 500 cycles and, in contrast, the extent ofirreversibility of Comparative Samples 6 a was 25.5% after just 100cycles.

Sample 6 f: The spray pyrolysis technique was applied to synthesize insitu a series of SnO₂-carbon nano-composites at 700° C. The in situspray pyrolysis process ensures that the chemical reaction is completedduring a short period of time, preventing the crystals from growing toolarge. A solution of SnCl₂.2H₂O and sucrose was used as a sprayprecursor. The spray precursors were prepared by mixing saturatedaqueous sucrose solutions with tin (II) chloride dehydrate (Aldrich,98%) 1 M ethanol solution, in SnCl₂.2H₂O/sucrose in weight ratios of100:0, 60:40, 40:60, and 10:90, respectively. The SnO₂ pure sample andSnO₂-carbon composites were obtained in situ using a vertical type ofspray pyrolysis reactor at 700° C. The process resulted in super finenanocrystalline SnO₂, which was distributed homogeneously inside theamorphous carbon matrix. The C—SnO₂ composite particles were dry-blendedwith approximately 23% NGP particles and then slightly compressed toobtain Sample 6 f. The anode material without NGPs was denoted asComparative Sample 6 f. The extent of irreversibility of Samples 6 f wasfound to be 6.7 and, in contrast, the extent of irreversibility ofComparative Samples 6 f was 27.5% after 100 cycles.

Example 7 Samples 7 a, 7 b and Comparative Samples 7 a, 7 b

Nanostructured CO₃O₄C and CO₃O₄ powders were prepared in separateexperiments using a vertical spray pyrolysis apparatus. The CO₃O₄powders was obtained by spraying aqueous 0.2 M cobalt nitrate solutionsat ambient temperature through an ultrasonic nozzle at 3 mL/min into anopen air 2 m quartz tube at 600° C. For the preparation of the Co₃O₄—Cpowder, the precursor solution contained 0.05 M sucrose and 0.2 M cobaltnitrate. The experimental conditions for pyrolysis of this solution wereidentical to those described for CO₃O₄ powder. The compositions of allfinal powder products were confirmed by X-ray diffraction (XRD). TheCO₃O₄—C and CO₃O₄ powders were separately dried blended with NGPs toobtain Samples 7 a and 7 b, respectively. The corresponding anodes,without NGPs but with a resin binder are denoted as Comparative Samples7 a and 7 b, respectively. The extent of irreversibility of Samples 7 awas found to be 6.3%, and, in contrast, the extent of irreversibility ofComparative Samples 6 a was 24.8% after 100 cycles. The extent ofirreversibility of Samples 7 b was found to be 6.4% and, in contrast,the extent of irreversibility of Comparative Samples 6 a was 27.3% after100 cycles.

In summary, the present invention provides an innovative, versatileplatform materials technology that enables the design and manufacture ofsuperior anode materials for lithium ion batteries or other types ofrechargeable batteries. This new technology appears to have thefollowing advantages:

-   -   (1) This approach is applicable to a wide spectrum of        electrochemically active materials, particularly those more        rigid materials or materials that undergo a large volume change        during absorption (or intercalation) and desorption (or        extraction) of lithium ions.    -   (2) The invented hybrid anode exhibits a reversible capacity        much higher than that of graphite (372 mAh/g) and a low        irreversible capacity loss.    -   (3) It seems that NGPs, being mechanically flexible, are capable        of accommodating or cushioning the strains or stresses induced        by volume changes during lithium insertion and extraction.    -   (4) With the active material particle size or coating thickness        being small (<200 μm, preferably less than 1 μm) and with NGP        length/width being small (preferably <5 μm), the distance for        lithium ions to travel is reduced. The anode can quickly store        or release lithium and thus can carry high current. This is a        highly beneficial feature for a battery that is intended for        high power density applications such as electric cars.    -   (5) The NGPs also serve to separate or isolate active material        particles from one another, preventing coalescence or sintering        of fine particles. Furthermore, with a sufficient amount of NGPs        that reach a state of percolation, the highly conductive        platelets form a continuous path for electrons, resulting in        significantly reduced internal energy loss or internal heating.

1. A nano-scaled graphene platelet-based composite composition for useas a lithium ion battery anode, said composition comprising: a) micron-or nanometer-scaled particles or coating which are capable of absorbingand desorbing lithium ions; and b) a plurality of separated or isolatednano-scaled graphene platelets (NGPs), wherein a platelet comprises agraphene sheet or a stack of graphene sheets having a platelet thicknessless than 100 nm; wherein at least one of said particles or coating isphysically attached or chemically bonded to at least one of saidplatelets and the amount of platelets is in the range of 2% to 90% byweight and the amount of particles or coating is in the range of 98% to10% by weight; wherein the particles or coating comprises an activematerial capable of absorbing or extracting lithium ions and said activematerial is selected from the group consisting of: (a) silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc(Zn), aluminum (Al), and cadmium (Cd); (b) alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements,wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (c) oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, or Cd, and their mixtures or composites; (d) salts and hydroxides ofSn; and (e) combinations thereof.
 2. The composite composition of claim1 wherein said NGPs are obtained from exfoliation and separation ofgraphene sheets of a laminar graphite material selected from naturalgraphite, synthetic graphite, highly oriented pyrolytic graphite,graphite fiber, carbon fiber, carbon nano-fiber, graphitic nano-fiber,spherical graphite or graphite globule, meso-phase micro-bead,meso-phase pitch, graphitic coke, graphitized polymeric carbon, or acombination thereof.
 3. The composite composition as defined in claim 1wherein said platelets have a specific surface area greater than about100 m²/gm or have an average thickness thinner than 10 nm.
 4. The hybridcomposition as defined in claim 1 wherein said platelets have a specificsurface area greater than about 500 m²/gm or have an average thicknessthinner than 2 nm.
 5. The composite composition as defined in claim 1wherein said particles have a dimension less than 5 μm or said coatinghas a thickness less than 5 μm.
 6. The composite composition as definedin claim 1, further comprising a conductive additive selected fromcarbon or graphitic nano-fiber, carbon nano-tube, carbon black,activated carbon powder, or a combination thereof.
 7. The compositecomposition as defined in claim 1 wherein the particles or coatingcomprise Sn or Si as a primary element with Si or Sn content no lessthan 20% by weight based on the total weight of the particles or coatingand nano-scaled graphene platelets.
 8. The composite composition asdefined in claim 1 wherein the particles comprise an element selectedfrom Si, Ge, Sn, Cd, Sb, Pb, Bi, or Zn.
 9. The composite composition asdefined in claim 1 wherein the particles or coating are amorphous orcomprise nano crystallites.
 10. The composite composition as defined inclaim 1 further comprising an amorphous carbon, polymeric carbon, carbonblack, coal tar pitch, petroleum pitch, or meso-phase pitch in physicalcontact with said particles or coating.
 11. The composite composition asdefined in claim 10 wherein said polymeric carbon is obtained frompyrolyzation of a polymer selected from the group consisting ofphenol-formaldehyde, polyacrylonitrile, styrene-based polymers,cellulosic polymers, epoxy resins, and combinations thereof.
 12. Thecomposite composition of claim 10 wherein said amorphous carbon phase isobtained from chemical vapor deposition, chemical vapor infiltration, orpyrolyzation of an organic precursor.
 13. A lithium secondary batterycomprising a positive electrode, a negative electrode comprising acomposite composition as defined in claim 1 which is capable ofabsorbing and desorbing lithium ions, and an electrolyte disposedbetween said negative electrode and said positive electrode.
 14. Thelithium secondary battery according to claim 13, wherein said positiveelectrode comprises lithium cobalt oxide, lithium nickel oxide, lithiummanganese oxide, lithium iron phosphate, lithium vanadium phosphate, ora combination thereof.
 15. The lithium secondary battery as defined inclaim 13, wherein said composite composition further comprises a bindermaterial selected from a polymer, coal tar pitch, petroleum pitch,meso-phase pitch, coke, or a derivative thereof.
 16. The lithiumsecondary battery as defined in claim 13, wherein said compositecomposition provides a specific capacity of no less than 600 mAh pergram of the anode composition.
 17. The lithium secondary battery asdefined in claim 13, wherein said composite composition provides aspecific capacity of no less than 1,000 mAh per gram of the anodecomposition.